This invention pertains generally to radar signal processing techniques, and, in particular, to an improved signal processing technique for enabling a low pulse repetition frequency (PRF) airborne radar to distinguish between target returns and returns from main beam clutter.
As is known, an aircraft intent on penetrating an enemy's airspace will fly at a very low altitude in order to avoid, or delay for the maximum possible time, detection by a ground-based enemy radar. Such penetrating aircraft may, however, be detected and interdicted by an interceptor aircraft utilizing a so-called "look-down, shoot-down" radar system which employs a forward and downward-looking radar beam. Generally, such radar systems employ a pulse Doppler radar operating at a relatively low pulse repetition frequency (PRF), with an antenna having a relatively small aperture. The problem of distinguishing between return signals attributable to main beam ground clutter and those from a moving target is encountered.
It is a common practice in operation of a pulse Doppler radar to change PRF and transmitted frequency from dwell to dwell so that in at least one dwell the problem of blind speeds is solved. Then, by range gating and passing all return signals through a Fast Fourier Transform (FFT) processor, target return signals and main beam clutter may be distinguished one from the other so that clutter frequency tracking loops and roughing filters may be controlled to reject the main beam clutter. Such loops and filters are, however, difficult to implement and are at times unreliable.
Another technique that may be used to separate main beam clutter from the target return signal is to calculate the ranges and Doppler frequencies at which main beam clutter should occur and then operate appropriate filters to reject any received signals that fall within the calculated ranges and Doppler frequencies. Such a technique, however, requires an accurate measurement of both attitude and velocity so that the accuracy of the requisite calculations may be maintained.